To date, production of olefins from light paraffins (ethane, propane and butane) have followed one of two major routes: non-catalytic thermal cracking in the presence of steam or catalytic dehydrogenation.
Ethylene plants are designed to maximize either ethylene or propylene or sum of ethylene and propylene. The thermal route produces a mixture of targeted products (ethylene, propylene, butadiene/butenes). The mixture will vary with the selected feed and the reaction conditions. The quantity of butadiene produced in the olefins plant is generally low. Thermal crackers operate at relatively high temperatures and hence the major product is ethylene.
On the other hand, dehydrogenation units, such as CATADIENE and CATOFIN units available from Lummus Technology LLC, dehydrogenate paraffins to olefins and olefins to dienes, and these units operate at relatively low temperatures. Hence, primary olefins (propylene from propane feed, normal butene from n-butane feed, and isobutene from isobutane feed) are the main products. When normal butenes are present in the feed, butadiene is also produced. Since the equilibrium ethane conversion is very low, the conversion of ethane to ethylene does not occur to a significant level. In addition, dehydrogenation reactions are carried out at low partial pressures either by operating the unit at low absolute pressure or in the presence of inert compounds. The feed to dehydrogenation has to be heated to 500 to 650° C. to carry out the reaction to achieve economical conversions.
Historically, the route for a particular plant has been chosen based on the available feed and the desired major product due to both energy and capital cost considerations. For example, with butanes as the feed, the thermal cracking route will produce about 42% ethylene as the majority product, but the catalytic dehydrogenation route will produce close to 60% butadiene/butene product. Although reaction conditions can be varied to change these results, they do not change the target product nor do they substantially change the yields.